U.S. patent application number 12/570579 was filed with the patent office on 2010-07-01 for method of detecting dust and method of preventing erroneous determination of dust detection.
This patent application is currently assigned to HONDA MOTOR CO., LTD.. Invention is credited to Tetsuro Hayashi, Masamitsu Numano, Junya Tanabe.
Application Number | 20100163532 12/570579 |
Document ID | / |
Family ID | 42234783 |
Filed Date | 2010-07-01 |
United States Patent
Application |
20100163532 |
Kind Code |
A1 |
Numano; Masamitsu ; et
al. |
July 1, 2010 |
METHOD OF DETECTING DUST AND METHOD OF PREVENTING ERRONEOUS
DETERMINATION OF DUST DETECTION
Abstract
A method of detecting dust includes: a first step of monitoring
a resistance value welding and determining if a variation amount of
the resistance value per unit time is equal to or more than a first
threshold; a second step of monitoring the resistance value after
the first step and determining if a variation amount of this
resistance value per unit time is equal to or less than a second
threshold; a third step of determining if a difference value
between a resistance value for calculating the variation amount
equal to or more than the first threshold and a resistance value
for calculating the variation amount determined equal to or less
than the second threshold is equal to or more than a third
threshold; and a fourth step of determining that dust is generated
when the difference value is equal to or more than the third
threshold.
Inventors: |
Numano; Masamitsu; (Hagagun,
JP) ; Tanabe; Junya; (Hagagun, JP) ; Hayashi;
Tetsuro; (Hagagun, JP) |
Correspondence
Address: |
RANKIN, HILL & CLARK LLP
38210 Glenn Avenue
WILLOUGHBY
OH
44094-7808
US
|
Assignee: |
HONDA MOTOR CO., LTD.
Tokyo
JP
|
Family ID: |
42234783 |
Appl. No.: |
12/570579 |
Filed: |
September 30, 2009 |
Current U.S.
Class: |
219/117.1 |
Current CPC
Class: |
B23K 11/34 20130101;
B23K 11/256 20130101 |
Class at
Publication: |
219/117.1 |
International
Class: |
B23K 11/25 20060101
B23K011/25 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 25, 2008 |
JP |
2008-329138 |
Feb 17, 2009 |
JP |
2009-033445 |
Claims
1. A method of detecting dust generation during resistance welding,
comprising: a first step of monitoring a resistance value during
the resistance welding and determining whether or not a variation
amount of the resistance value per unit time is equal to or more
than a first threshold; a second step of monitoring the resistance
value during the resistance welding after the first step and
determining whether or not a variation amount of the resistance
value per unit time is equal to or less than a second threshold; a
third step of determining whether of not a difference value between
a resistance value for calculating the variation amount determined
in the first step as being equal to or more than the first
threshold and a resistance value for calculating the variation
amount determined in the second step as being equal to or less than
the second threshold is equal to or more than a third threshold;
and a fourth step of determining that dust is generated when the
difference value is equal to or more than the third threshold in
the third step.
2. A method of preventing erroneous determination of dust detection
preventing an erroneous determination of a device monitoring a
resistance value during resistance welding and detecting dust
generation when a variation amount of the resistance value per unit
time is equal to or more than a threshold, comprising: a first step
of setting an upper limit on a resistance value at a start of
normal resistance welding as an upper limit threshold; a second
step of determining whether or not the resistance value during
resistance welding exceeds the upper limit threshold; and a third
step of, when the resistance value exceeds the upper limit
threshold, not performing operation on dust detection during a time
when the resistance value exceeds the upper limit threshold and
during a predetermined time after the resistance value returns to
be equal to or less than the upper limit threshold.
Description
[0001] This application is based on and claims the benefit of
priority from Japanese Patent Applications Nos. 2008-329138 and
2009-033445, respectively filed on 25 Dec. 2008 and 17 Feb. 2009,
the content of which is incorporated herein by reference.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates to a method of detecting dust
and a method of preventing erroneous determination of dust
detection during resistance welding such as spot welding.
[0004] 2. Related Art
[0005] According to the related art, a lot of welding robots which
spot-weld the predetermined part of a work conveyed by a conveyance
device are arranged in a production line for welding car bodies and
assembling thereof. These welding robots are provided with a spot
welding gun having a shape appropriate for the predetermined
welding part of a work.
[0006] These kind of welding robots monitor a resistance value
while a welding current is applied. When dust is generated while a
welding current is applied, a considerably decreased resistance
value is monitored.
[0007] In the case in which such dust is generated, the strength of
the welding part may be insufficient even after a predetermined
welding current is applied for a predetermined current applying
time. This is because the growing rate of a nugget decreases by the
generated dust, and the size of the nugget becomes small.
[0008] Thus, welding robots are programmed so as to determine that
dust is generated and perform a predetermined process on dust
detection to shut off, increase, or decrease a welding current when
the resistance value decreases while a welding current is applied
(refer to Unexamined Japanese Patent Application, First Publication
No. 2006-55893).
[0009] By the way, when welding operation is performed, for
example, under condition in which the spot welding guns of two
adjacent welding robots are close to each other, the magnetic field
of the first spot welding gun affects that of the second spot
welding gun, so that the resistance value may temporarily
decreases. Such the decrease of the resistance value is temporal,
which is a noise fading out at once, not affecting welding
quality.
[0010] However, on such noise generation, welding robots may
erroneously determine that dust is generated due to the decrease of
the resistance value. When erroneously determining that dust is
generated in this way, welding robots perform a predetermined
process on dust detection to shut off, increase, or decrease a
welding current.
[0011] The process on dust detection based on such erroneous
determination is performed, causing improper operation such as
uselessly stopping welding operation and setting a welding current
to an abnormal value.
[0012] In addition, when foreign matter such as dust enters between
material members to be welded, the initial resistance value for
resistance welding extremely increases compared with that at the
start of normal resistance welding. Then, the foreign matter melts,
so that the initial resistance value may rapidly decrease. In this
case, welding robots erroneously determine that dust is generated
due to the rapid decrease of the initial resistance value and
perform a predetermined process on dust detection to shut off,
increase, or decrease a welding current.
[0013] The process on dust detection based on such erroneous
determination is performed, causing improper operation such us
uselessly stopping welding operation and setting a welding current
to an abnormal value.
SUMMARY OF THE INVENTION
[0014] It is an object of the present invention to provide a method
of detecting dust which is capable of preventing erroneous
determination of dust generation determined by noise generation and
preventing erroneous operation performing a process on dust
detection based on the erroneous determination.
[0015] It is another object of the present invention to provide a
method of preventing erroneous determination of dust detection
which is capable of preventing erroneous determination of dust
generation determined by the rapid decrease of a resistance value
which is caused by entered foreign matter such as dust and
preventing erroneous operation performing a process on dust
detection based the erroneous determination.
[0016] The method of detecting dust of the present invention
detects dust generation during resistance welding, including: a
first step (for example, the below-mentioned step S2 shown in FIG.
2) of monitoring a resistance value (for example, the
below-mentioned resistance value r) during the resistance welding
and determining whether or not a variation amount of the resistance
value per unit time (for example, the below-mentioned variation
amount rd of the resistance value) is equal to or more than a first
threshold (for example, the below-mentioned first threshold T1); a
second step (for example, the below-mentioned step S4 shown in FIG.
2) of monitoring the resistance value during the resistance welding
after the first step and determining whether or not a variation
amount of the resistance value per unit time is equal to or less
than a second threshold (for example, the below-mentioned second
threshold T2); a third step (for example, the below-mentioned step
S5 shown in FIG. 2) of determining whether or not a difference
value (for example, the below-mentioned difference value rD)
between a resistance value (for example, the below-mentioned
resistance value rm) for calculating the variation amount (for
example, the below-mentioned variation amount rd5 of the resistance
value) determined in the first step as being equal to or more than
the first threshold and a resistance value (for example, the
below-mentioned resistance value r10) for calculating the variation
amount (for example, the below-mentioned variation amount rd10 of
the resistance value) determined in the second step as being equal
to or less than the second threshold is equal to or more than a
third threshold (for example, the below-mentioned third threshold
T3); and a fourth step (for example, the below-mentioned step S6
shown in FIG. 2) of determining that dust is generated when the
difference value is equal to or more than the third threshold in
the third step.
[0017] Accordingly, the present invention can prevent erroneous
determination of dust generation determined by noise generation and
erroneous operation performing a process on dust detection based
the erroneous determination.
[0018] The method of preventing erroneous determination of dust
detection of the present invention prevents an erroneous
determination of a device monitoring a resistance value (for
example, the below-mentioned resistance value r) during resistance
welding and detecting dust generation when a variation amount of
the resistance value per unit time (for example, the
below-mentioned variation amount rd of the resistance value) is
equal to or more than a threshold (for example, the below-mentioned
threshold L), including: a first step (for example, the
below-mentioned step S1 shown in FIG. 5) of setting an upper limit
on a resistance value at the start of normal resistance welding as
an upper limit threshold (for example, the below-mentioned upper
limit threshold UL); a second step (for example, the
below-mentioned step S3 shown in FIG. 5) of determining whether or
not the resistance value during resistance welding exceeds the
upper limit threshold; and a third step (for example, the
below-mentioned step S6 shown in FIG. 5) of, when the resistance
value exceeds the upper limit threshold, not performing operation
on dust detection during a time when the resistance value exceeds
the upper limit threshold and during a predetermined time (for
example, the below-mentioned predetermined time T) after the
resistance value returns to be equal to or less than the upper
limit threshold.
[0019] Accordingly, the present invention can prevent erroneous
determination of dust generation determined by the rapid decrease
of a resistance value caused by entered foreign matter such as dust
and prevent erroneous operation performing a process on dust
detection based the erroneous determination.
BRIEF DESCRIPTION OF THE DRAWINGS
[0020] FIG. 1 is an outline explanatory drawing of a spot welding
device performing the method of detecting dust according to a first
embodiment of the present invention;
[0021] FIG. 2 is a flow chart of one embodiment of the method of
detecting dust according to the first embodiment;
[0022] FIG. 3 is a timing chart illustrating the relationship
between the time of applying a welding current and the resistance
value in the method of detecting dust according to the first
embodiment;
[0023] FIG. 4 is an outline explanatory drawing of a spot welding
device performing the method of preventing erroneous determination
of dust detection according to a second embodiment of the present
invention;
[0024] FIG. 5 is a flow chart of one embodiment of the method of
preventing erroneous determination of dust detection according to
the second embodiment;
[0025] FIG. 6 is a detailed flow chart of the step S7 shown in FIG.
5; and
[0026] FIG. 7 is a timing chart illustrating the relationship
between the time of applying a welding current and the
interelectrode resistance value in the method of preventing
erroneous determination of dust detection according to the second
embodiment.
DETAILED DESCRIPTION OF THE INVENTION
[0027] Hereinafter, the first embodiment of the present invention
will be explained with reference to the drawings. FIG. 1 is an
outline explanatory drawing of a spot welding device 10 performing
the method of detecting dust according to the present invention.
The spot welding device 10 is provided with a pair of electric tips
3 and 4 pressurizing accumulated material members 1 and 2 to be
welded and applying a welding current thereto, a pressure device
(not shown) applying pressure to the electric tips 3 and 4, a power
supply device (not shown) supplying a welding current to the
material members 1 and 2 pressurized by the pressure device through
the electric tips 3 and 4, and a control unit 5 controlling
thereof.
[0028] In the spot welding device 10 configured in this way, a
welding current predetermined depending on a welding condition is
applied to the material members 1 and 2 through the electric tips 3
and 4 for only a predetermined current applying time to grow a
nugget 6 at a predetermined rate, thereby performing welding.
[0029] The control unit 5 detects dust generation in accordance
with the flow chart of FIG. 2. A welding current for spot welding
starts to be applied, and the control unit 5 starts to monitor the
resistance value in the step S1. The resistance value is detected
at every predetermined time interval. Specifically, the resistance
value which the control unit 5 detects at the time t1 is defined as
r1, and the resistance value which the control unit 5 detects at
the time t2 a unit time after the time t1 is defined as r2. The
control unit 5 repeats to detect the resistance value from the
start of spot welding to the end at every unit time interval.
[0030] In the step S2, the control unit 5 is determined whether or
not the variation amount of the monitored resistance value per unit
time is equal to or more than the first threshold. The control unit
5 detects the resistance value at every unit time interval to
calculate the variation amount of the resistance value per unit
time based on each of the resistance values at any consecutive two
points. Then, the control unit 5 determines whether or not the
calculated variation amount of the resistance value per unit time
is equal to or more than the first threshold.
[0031] This first threshold represents the reduced amount of the
resistance value decreased by dust generation in the variation
amount of the resistance value per unit time. If this determination
is "YES", the resistance value for calculating the variation amount
is stored, and the process proceeds to the step S3. If this
determination is "NO", the process proceeds to the step S8.
[0032] In the step S3, the control unit 5 determines that dust may
be generated.
[0033] In the step S4, the control unit 5 determines whether or not
the variation amount of the monitored resistance value per unit
time is equal to or less than the second threshold.
[0034] This second threshold represents the reduced amount of the
resistance value converged by the end of dust generation in the
variation amount of the resistance value per unit time. If this
determination is "YES", the process proceeds to the step S5. If
this determination is "NO", the process returns to the step S4.
Thus, the determination of the step S4 is repeated until the
variation amount of the resistance value per unit time becomes
equal to or less than the second threshold.
[0035] In the step S5, the control unit 5 reads out the resistance
value for calculating the variation amount determined in the step
S2 as being equal to or more than the first threshold and
determines that the difference value between this resistance value
and the resistance value for calculating the variation amount
determined in the step S4 as being equal to or less than the second
threshold is equal to or more than the third threshold.
[0036] This third threshold is used for distinguishing the reduced
amount of the resistance value decreased by dust generation from
the reduced amount of the resistance value decreased by noise
generation. For example, the third threshold T3 is set to 5-50
.mu..OMEGA.. If this determination is "YES", the process proceeds
to the step S6. If this determination is "NO", the process proceeds
to the step S8.
[0037] In the step S6, the control unit 5 determines that dust is
generated.
[0038] In the step S7, the control unit 5 performs the process on
dust detection. Specifically, the control unit 5 performs the
process preprogrammed as the process on dust detection, for
example, to shut off, increase, or decrease a welding current.
After the predetermined process on dust detection completes,
welding operation ends.
[0039] In the step S8, the control unit 5 determines that no dust
is generated.
[0040] Then, in the step S9, the control unit 5 determines whether
or not welding has ended. Specifically, the control unit 5 refers
to the predetermined welding program to determine whether or not
welding has ended.
[0041] If welding has not ended, the determination of the step S9
is "NO", and the process returns to the step S2. In the case in
which the determination of the step S5 is "NO", the process
proceeds to the step S8 followed by the step S9, the determination
of the step S9 is "NO", and the process returns to the step S2, the
resistance value is continuously monitored until the end of
welding, since noise is generated during welding.
[0042] While the process in which the determination of the step S2
is "NO", the process proceeds to the step S8 followed by the step
S9, the determination of the step S9 is "NO", and the process
returns to the step S2 is repeated, the state in which the
variation amount of the resistance value per unit time is less than
the first threshold is maintained. Thus, normal welding operation
without dust and noise generation is performed.
[0043] Then, when welding ends, the determination of the step S9
becomes "YES", and welding operation ends normally.
[0044] FIG. 3 is a timing chart illustrating the relationship
between the time of applying a welding current and the resistance
value in the method of detecting dust according to the first
embodiment. In FIG. 3, the continuous line illustrates the
relationship between the current applying time and the resistance
value when welding operation is not normally performed since dust
is generated during welding operation. For comparison, the dashed
line illustrates the relationship between the current applying time
and the resistance value when dust is generated during welding
operation but welding operation is practically and normally
performed. In addition, the two-dot chain line illustrates the
relationship between the current applying time and the resistance
value when welding operation is normally performed.
[0045] In FIG. 3, the dust generation illustrated by the continuous
line and the noise generation illustrated by the dashed line have
in common because the resistance value falls by dust or noise
generation, but in contrast because there are a significant
difference in the reduced amount of the resistance value.
Specifically, in the case of the dust generation illustrated by the
continuous line, the resistance value falls more sharply and does
not return to a normal level after falling. On the other hand, in
the case of the noise generation illustrated by the dashed line,
the resistance value falls less sharply and returns to a normal
level immediately after falling.
[0046] As the continuous line illustrates in FIG. 3, determination
is made as follows in accordance with the flow chart shown in FIG.
2 when dust is generated during welding operation. Specifically,
the control unit 5 starts to monitor the resistance value. This
allows the control unit 5 to detect the resistance value r2 at
every unit time interval. The resistance value r1 is detected at
the time t1, and the resistance value r2 is detected at the time t2
a unit time later. Then, the difference value between the
resistance value r1 and the resistance value r2 is calculated as
the variation amount rd1 of the resistance value. The variation
amount rd1 of the resistance value is not equal to or more than the
first threshold T1, so that it is determined that no dust is
generated. The resistance value r3 is detected at the time t3 a
unit time later. Then, the difference value between the resistance
value r2 and the resistance value r3 is calculated as the variation
amount rd2 of the resistance value. The variation amount rd2 of the
resistance value is not equal to or more than the first threshold
T1, so that it is determined that no dust is generated. The
resistance values r4 and r5 are respectively detected at the times
t4 and t5 a unit time later. Then, the difference value between the
resistance value r3 and the resistance value r4 is calculated as
the variation amount rd3 of the resistance value, and the
difference value between the resistance value r4 and the resistance
value r5 is calculated as the variation amount rd4 of the
resistance value. The variation amounts rd3 and rd4 of the
resistance value are not equal to or more than the first threshold
T1, so that it is determined that no dust is generated.
[0047] The resistance value r6 is detected at the time t6 a unit
time later. Then, the difference value between the resistance value
r5 and the resistance value r6 is calculated as the variation
amount rd5 of the resistance value. The variation amount rd5 of the
resistance value is equal to or more than the first threshold T1,
so that it is determined that dust may be generated.
[0048] The resistance value r7 is detected at the time t7 a unit
time later. Then, the difference value between the resistance value
r6 and the resistance value r7 is calculated as the variation
amount rd6 of the resistance value. The variation amount rd6 of the
resistance value is not equal to or less than the second threshold
T2. The resistance values r8, r9, and r10 are respectively detected
at the times t8, t9, and t10 a unit time later. Then, the
difference value between the resistance value r7 and the resistance
value r8 is calculated as the variation amount rd7 of the
resistance value, the difference value between the resistance value
r8 and the resistance value r9 is calculated as the variation
amount rd8 of the resistance value, and the difference value
between the resistance value r9 and the resistance value r10 is
calculated as the variation amount rd9 of the resistance value. The
variation amounts rd7, rd8, and rd9 of the resistance value are not
equal to or less than the second threshold T2.
[0049] The resistance value r11 is detected at the time t11 a unit
time later. Then, the difference value between the resistance value
r10 and the resistance value r11 is calculated as the variation
amount rd10 of the resistance value. The variation amount rd10 of
the resistance value is equal to or less than the second threshold
T2.
[0050] At this point, the difference value rD between the
resistance value rm for calculating the variation amount rd5 of the
resistance value at the time t6 and the resistance value r10 for
calculating the variation amount rd10 of the resistance value at
the time t11 is calculated because the variation amount rd10 of the
resistance value becomes equal to or less than the second threshold
T2. This difference value rD is the difference value rDs equal to
or more than the third threshold 13, so that it is determined that
dust is generated.
[0051] As the dashed line illustrates in FIG. 3, determination is
made as follows in accordance with the flow chart shown in FIG. 2
when noise is generated during welding operation. Specifically, the
control unit 5 starts to monitor the resistance value. The
resistance values r1, r2, r3, r4, and r5 are detected at the times
t1, t2, t3, t4, and t5 respectively. Then, the difference value
between the resistance value r1 and the resistance value r2 is
calculated as the variation amount rd1 of the resistance value, the
difference value between the resistance value r2 and the resistance
value r3 is calculated as the variation amount rd2 of the
resistance value, the difference value between the resistance value
r3 and the resistance value r4 is calculated as the variation
amount rd3 of the resistance value, and the difference value
between the resistance value r4 and the resistance value r5 is
calculated as the variation amount rd4 of the resistance value. The
variation amounts rd1, rd2, rd3, and rd4 of the resistance value
are not equal to or more than the first threshold T1, so that it is
determined that no dust is generated.
[0052] The resistance value r6 is detected at the time t6 a unit
time later. Then, the difference value between the resistance value
r5 and the resistance value r6 is calculated as the variation
amount rd5 of the resistance value. The variation amount rd5 of the
resistance value is equal to or more than the first threshold T1,
so that it is determined that dust may be generated.
[0053] The resistance values (r7), (r8), and (r9) are detected at
the times t7, t8, and t9 respectively. Then, the difference value
between the resistance value r6 and the resistance value (r7) is
calculated as the variation amount (rd6) of the resistance value,
the difference value between the resistance value (r7) and the
resistance value (r8) is calculated as the variation amount (rd7)
of the resistance value, and the difference value between the
resistance value (r8) and the resistance value (r9) is calculated
as the variation amount (rd8) of the resistance value. The
variation amounts (rd6), (rd7), and (rd8) of the resistance value
are not equal to or less than the second threshold T2.
[0054] The resistance value (r10) is detected at the time t10 a
unit time later. Then, the difference value between the resistance
value (r9) and the resistance value (r10) is calculated as the
variation amount (rd9) of the resistance value. The variation
amount (rd9) of the resistance value is equal to or less than the
second threshold T2.
[0055] At this point, the difference value rD between the
resistance value rm for calculating the variation amount rd5 of the
resistance value at the time t6 and the resistance value (r9) for
calculating the variation amount (rd9) of the resistance value at
the time t10 is calculated because the variation amount (rd9) of
the resistance value becomes equal to or less than the second
threshold T2. This difference value rD is the difference value rDn
less than the third threshold T3, so that it is determined that no
dust is generated.
[0056] The present embodiment has the following effects.
[0057] (1) The control unit 5 can prevent erroneous determination
of dust generation determined by noise generation and erroneous
operation performing a process on dust detection based the
erroneous determination.
[0058] (2) The control delay is not occurred because the difference
value rDs is calculated, thereby determining that dust is
generated, at the same time when the resistance value r11 is
detected at the time t11.
[0059] Hereinafter, the second embodiment of the present invention
will be explained with reference to the drawings. FIG. 4 is an
outline explanatory drawing of a spot welding device 10 performing
the method of preventing erroneous determination of dust detection
according to the present invention.
[0060] The spot welding device 10 is provided with a pair of
electric tips 3 and 4 pressurizing accumulated material members 1
and 2 to be welded and applying a welding current thereto, a
pressure device (not shown) applying pressure to the electric tips
3 and 4, a power supply device (not shown) supplying a welding
current to the material members 1 and 2 pressurized by the pressure
device through the electric tips 3 and 4, and a control unit 5
controlling thereof.
[0061] In the spot welding device 10 configured in this way, a
welding current predetermined depending on a welding condition is
applied to the material members 1 and 2 through the electric tips 3
and 4 for only a predetermined current applying time to grow a
nugget 6 at a predetermined rate, thereby performing welding.
[0062] The control unit 5 detects dust generation in accordance
with the flow chart of FIGS. 5 and 6. In the flow chart of FIG. 5,
the control unit 5 first defines the upper limit of the range of
the interelectrode resistance as the upper limit threshold in the
step S1. Hereinafter, "the range of the interelectrode resistance"
is the usual range of the interelectrode resistance at the moment
of the start of welding material members under normal conditions,
for example, in which foreign matter is not entered, by resistance
welding. Specifically, the upper limit threshold is set to 50-500
.mu..OMEGA..
[0063] In the step S2, the control unit 5 starts to monitor the
resistance value. The resistance value is detected at every
predetermined time interval. Specifically, the resistance value
which the control unit 5 detects at the time t1 is defined as r1,
and the resistance value which the control unit 5 detects at the
time t2 a unit time after the time t1 is defined as r2. The control
unit 5 repeats to detect the resistance value from the start of
spot welding to the end at every unit time interval.
[0064] In the step S3, the control unit 5 determines whether or not
the monitored resistance value exceeds the upper limit threshold.
If this determination is "NO", the number of times of the
determinations made in the step S3 is stored, and the process
proceeds to the step S4. If this determination is "YES", the
process proceeds to the step S6.
[0065] In the step S4, the control unit 5 determines whether or not
the number of times of the determinations made in the step S3 is
one. Specifically, the control unit 5 determines whether or not it
is determined that the resistance value is equal to or less than
the upper limit threshold at the first determination of the step
S3. If this determination is "NO", the process proceeds to the step
S5. If this determination is "YES", the process returns to the step
S7.
[0066] In the step S5, the control unit 5 determines whether or not
the predetermined time has passed since the monitored resistance
value became equal to or less than the upper limit threshold. In
this case, the resistance value monitored by the control unit 5
once exceeded the upper limit threshold, but returns to be equal to
or less than the upper limit threshold later. Therefore, the time
until the resistance value become equal to that at the start of
normal welding is defined as the predetermined time after the
resistance value becomes equal to or less than the upper limit
threshold.
[0067] This predetermined time varies depending on the types of
material members to be welded. It also varies depending on entered
foreign matter such as dust. The foreign matter includes sealer,
dust, and the like. The resistance value may fall sharply at the
moment that material members such as high tension steel sheets with
a wide gap fit and contact to each other. Therefore, the
predetermined time is previously determined in accordance with the
characteristics of material members to be welded from experiment.
In the present embodiment, the predetermined time is set to 2-50
microseconds. If the determination of the step S5 is "NO", the
process proceeds to the step S6. If this determination is "YES",
the process returns to the step S7.
[0068] In the step S6, the control unit 5 does not perform the
operation on dust detection. If the determination of the step S3 is
"YES", the process returns to the step S3 without the operation on
dust detection performed because the resistance value monitored by
the control unit 5 exceeds the upper limit threshold.
[0069] If the determination of the step S5 is "NO", the process
returns to the step S3 without the operation on dust detection
performed because the resistance value monitored by the control
unit 5 once exceeded the upper limit threshold, but returns to be
equal to or less than the upper limit threshold later, yet the time
until the resistance value become equal to that at the start of
normal welding has not passed after the resistance value became
equal to or less than the upper limit threshold.
[0070] Specifically, the process returns to the step S3 without the
operation on dust detection performed while the resistance value
monitored by the control unit 5 exceeds the upper limit threshold,
and until the resistance value become equal to that at the start of
normal welding after the resistance value monitored by the control
unit 5 returns to be equal to or less than the upper limit
threshold.
[0071] In the step S7, the control unit 5 performs the operation on
dust detection.
[0072] At this point, the resistance value monitored by the control
unit 5 is equal to or less than the upper limit threshold from the
start of welding.
[0073] The operation on dust detection in the step S7 will be
explained with reference to the flow chart of FIG. 6.
[0074] In the step S71, the control unit 5 is determined whether or
not the variation amount of the monitored resistance value per unit
time is equal to or more than a threshold. The control unit 5
detects the resistance value at every unit time interval to
calculate the variation amount of the resistance value per unit
time based on each of the resistance values at any consecutive two
points. Then, the control unit 5 determines whether or not the
calculated variation amount of the resistance value per unit time
is equal to or more than the threshold.
[0075] This threshold represents the reduced amount of the
resistance value decreased by dust generation in the variation
amount of the resistance value per unit time. If this determination
is "YES", the process proceeds to the step S72. If this
determination is "NO", the process proceeds to the step S74.
[0076] In the step S72, the control unit 5 determines that dust is
generated.
[0077] In the step S73, the control unit 5 performs the process on
dust detection. Specifically, the control unit 5 performs the
process preprogrammed as the process on dust detection, for
example, to shut off, increase, or decrease a welding current.
After the predetermined process on dust detection completes,
welding operation ends.
[0078] In the step S74, the control unit 5 determines that dust is
not generated.
[0079] Then, in the step S75, the control unit 5 determines whether
or not welding has ended. Specifically, the control unit 5 refers
to the predetermined welding program to determine whether or not
welding has ended.
[0080] If welding has not ended, the determination of the step S75
is "NO", and the process returns to the step S71. While the process
in which the determination of the step S71 is "NO", the process
proceeds to the step S74 followed by the step S75, the
determination of the step S75 is "NO", and the process returns to
the step S71 is repeated, the state in which the variation amount
of the resistance value per unit time is less than the threshold is
maintained. Thus, normal welding operation without dust generation
is performed.
[0081] Then, when welding ends, the determination of the step S75
becomes "YES", and welding operation ends normally.
[0082] FIG. 7 is a timing chart illustrating the relationship
between the time of applying a welding current and the
interelectrode resistance value in the method of preventing
erroneous determination of dust detection according to the second
embodiment. In FIG. 7, the continuous line illustrates the
relationship between the current applying time and the
interelectrode resistance value when the interelectrode resistance
value is abnormally high at the start of welding operation by
foreign matter such as dust entered between material members to be
weld, and the resistance value decreases to a normal value by
melted foreign matter. The dashed line illustrates the relationship
between the current applying time and the interelectrode resistance
value when welding operation is not normally performed since dust
is generated during welding operation, for comparison. In addition,
the two-dot chain line illustrates the relationship between the
current applying time and the interelectrode resistance value when
welding operation is normally performed.
[0083] As the continuous line illustrates in FIG. 7, determination
is made as follows in accordance with the flow chart shown in FIG.
5 when dust is entered at the start of welding operation.
Specifically, the control unit 5 detects the resistance value r at
every unit time interval.
[0084] The resistance value r1 is detected at the time t1. It is
determined that the operation on dust detection is not performed
because the resistance value r1 exceeds the upper limit threshold
UL. The resistance value r2 is detected at the time t2 a unit time
later. The resistance value r3 is detected at the time t3 a unit
time later. It is determined that the operation on dust detection
is not performed because the resistance values r2 and r3 exceed the
upper limit threshold UL.
[0085] The resistance value r4 is detected at the time t4 a unit
time later. At this point, the resistance value r4 is equal to the
upper limit threshold UL, but the predetermined time T has not
passed since the resistance value r4 became equal to the upper
limit threshold UL, and it is thus determined that the operation on
dust detection is not performed.
[0086] The resistance value r5 is detected at the time t5. The
resistance value r5 is equal to the resistance value at the start
of normal welding. In this case, it is determined that the
operation on dust detection is performed because the elapsed time
from the time t4 to the time t5 is equal to the predetermined time
T. Thereafter, the operation on dust detection is repeated at every
unit time interval until welding ends.
[0087] FIG. 7 shows the length of the predetermined time T until
the resistance value monitored by the control unit 5 becomes equal
to the resistance value at the start of normal welding after it
becomes equal to or less than the upper limit threshold is equal to
that of every unit time interval at which the control unit 5
detects the resistance value r. However, there is no correlation
between the predetermined time T and the unit time interval, the
lengths of which are varied.
[0088] As the continuous line illustrates in FIG. 7, determination
is made as follows in accordance with the flow chart shown in FIG.
6 when dust is generated during welding operation. The explanation
regarding the period between the time t1 and the time t5 is
omitted.
[0089] The resistance value (r6) is detected at the time t6, and
the resistance value (r7) is detected at the time t7 a unit time
later. Then, the difference value between the resistance value (r6)
and the resistance value (r7) is calculated as the variation amount
rd6 of the resistance value. The variation amount rd6 of the
resistance value is not equal to or more than the threshold L, so
that it is determined that no dust is generated.
[0090] The resistance value (r8) is detected at the time t8 a unit
time later. Then, the difference value between the resistance value
(r7) and the resistance value (r8) is calculated as the variation
amount rd7 of the resistance value. The variation amount rd7 of the
resistance value is equal to or more than the threshold L, so that
it is determined that dust is generated.
[0091] The present embodiment has the following effects.
[0092] (3) The control unit 5 can prevent from erroneously
determining that dust is generated because of sharp decrease of the
resistance value due to entered foreign matter, by not performing
the operation on dust detection while the resistance value is
abnormal due to entered foreign matter such as dust.
[0093] (4) The control unit 5 can prevent improper operation such
as uselessly stopping welding operation and setting a welding
current to an abnormal value, which caused by the process performed
on dust generation to shut off, increase, or decrease a welding
current based on the erroneous determination of dust
generation.
[0094] (5) Welding failure caused by erroneous operation as a
processing result based on the erroneous determination of dust
generation can be prevented from occurring.
[0095] While preferred embodiments of the present invention have
been described and illustrated above, it is to be understood that
they are exemplary of the invention and are not to be considered to
be limiting. Additions, omissions, substitutions, and other
modifications can be made thereto without departing from the spirit
or scope of the present invention. Accordingly, the invention is
not to be considered to be limited by the foregoing description and
is only limited by the scope of the appended claims.
* * * * *